(i) Field of the Invention
The subject of the invention is a process for preparing a solid composite electrolyte consisting of at least one compound of the BIMEVOX family and the use of said composite for extracting oxygen from air or from a gas mixture containing oxygen.
(ii) Description of Related Art
In order to separate oxygen from air or from a gas mixture containing it, by means of a solid-electrolyte cell or membrane, whether using a purely electrochemical process generated by a current generator or using an electrochemical semipermeability process generated by the difference in oxygen partial pressure between each of the faces of the cell or membrane, the solid electrolytes used are generally stabilized zirconias, such as those described for example in United States Patent published under the number U.S. Pat. No. 4,879,016, or oxides derived from Bi.sub.4 V.sub.2 O.sub.11, such as those described in the International Patent Application published under the number WO 91/01274 and known by the generic name BIMEVOX. The latter, in which a variable fraction of the vanadium is replaced with a metal chosen especially from alkaline-earth metals, transition metals, rare earths or elements of Groups III to V of the Periodic Table of the Elements, are O.sup.2- ionic conductors and their anionic conductivity at 500.degree. C. is of the same order of magnitude as that of stabilized zirconias at 800.degree. C. The partial substitution of the constituent elements of Bi.sub.4 V.sub.2 O.sub.11 stabilizes the gamma-phase structural type and maintains, in the O.sup.2- ion lattice, a vacancy content sufficient to allow anionic conductivity. Since the oxygen atoms of the Bi.sub.2-x M.sub.y O.sub.2 layers strongly bonded to the bismuth atoms cannot move, it therefore means that the conductivity is essentially two-dimensional; the anionic conductivity of this phase is remarkable since it reaches 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 at about 200.degree. C.
These BIMEVOX compounds are used in electrochemical cells, like those described in the International Patent Applications published under numbers WO 94/06544 and WO 94/06545, and in cells operating by electrochemical semipermeability, such as those described in French Patent Application published under the number FR 2,698,016.
However, obtaining the intrinsic properties of these BIMEVOX-based electrolytes, whether mechanical or electrical properties, is conditional on proper control of the parameters for their synthesis. The term "synthesis parameters" should be understood to mean essentially the size of the initial particles and, during sintering the temperatures, rates of temperature rise and fall and the durations of any temperature hold at a given temperature. Likewise, depending on the metal chosen as the dopant and on its content, between 0% and 50%, it is necessary for the sintering conditions to be defined very precisely. These conditions have a direct influence on the particle size of the sintered material, this size itself governing the stability and the quality of the electrical and mechanical properties of the BIMEVOX compound. It has also been found that increasing the sintering temperatures by a few tens of degrees, for example 800.degree. C. instead of 750.degree. C. in the case of Bi.sub.2 Co.sub.0.1 V.sub.0.9 O.sub.3.55, called hereafter BICOVOX. 10, has the effect of modifying the electrical properties of the material and that too great a temperature in this has the effect of favoring abnormal crystalline grain growth in BIMEVOX, therefore leading to a microstructure in the inhomogeneous material in which the average grain size exceeds the critical value of 5 .mu.m. Under these conditions, on the one hand the mechanical properties degrade and the electrical resistivity increases, and on the other hand the electrical properties are no longer either stable over time or during thermal cycles.
In addition, poor adhesion of the electrode to the electrolyte in an elementary electrochemical cell of the BIMEVOX electrolyte/volume electrode type, such as the one described in the International Patent Application published under the number WO 95/32050, may be observed, and debonding phenomena have been demonstrated during operation of the cell. Thus, it has been possible to show that the adhesion of a composite electrode of the LSM-Bi.sub.2 Co.sub.0.1 V.sub.0.9 O.sub.5.35 type (30% by volume) to an electrolyte of composition Bi.sub.2 Co.sub.0.1 O.sub.5.35 after heating at 750.degree. C., which is the maximum temperature for preventing abnormal grain growth of the electrolyte, is very poor.
The Applicant has therefore sought to develop a process for preparing a solid electrolyte comprising at least one compound of the BIMEVOX family which allows grain size of the material after sintering to be controlled and which thus results in mechanical properties that are superior compared to said BIMEVOX material alone, without the electronic and ionic properties of said material thereby suffering.